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Thanks to Paul Hirose for a really interesting posting about the
astro-inertial system for the B-2.

I appreciate that Paul may find himself unable to elaborate on the details,
but some questions do come to mind about achieving the levels of precision
claimed in the document, so it may be worth some discussion and perhaps
speculation. I should add that I have no expertise at all in this field.

As some of the paragraphs are written in the future tense, it may be that
some of the reported accuracies were aims rather than achievements. Please
forgive any cynicism on my part, but the claim "Accuracies corresponding to
15 to 30 meters on the Earth's surface are attained by automated celestial
systems, depending on the degree of automation." is mind-boggling to say
the least!

If that claim related simply to the accuracy of the calculations, then it
would be easy to accept. It's rather harder to understand if it relates to
the overall precision of the complete system, which would correspond to
measuring star altitudes, with respect to the vertical, to 0.5 to 1 second
of arc

Not so difficult to define the direction of a star to that precision,
perhaps, but what worries me more are the errors in establishing the
direction of the vertical reference. An aircraft is different from a
spacecraft in that it has some unavoidable degree of buffeting from the
atmosphere that it's flying in, small changes in engine output, tiny
movements of control surfaces, movement of the crew, together with
gravitational anomalies from the ground it's passing over.

Establishing the direction of the gravitational vertical to within 0.5
second of arc involves knowing, or somehow compensating for, the aircraft's
accelerations to the order of two parts in a million of gravity. No doubt,
this is one of the jobs the inertial navigation system has to do, but then
it's stated below that- "the ACN will provide stabilization for both
position and platform alignment." so it appears that at least some aspects
of the aircraft's orientation are taken from the Advanced Celestial
Navigator, rather than supplied to it. If the nav platform is orienting
itself from star directions, then how are those star directions measured
with respect to the gravity vertical?

There must have been serious technical problems to solve in putting such a
system together, and if the claimed accuracy was indeed achieved in
real-life then some team of system engineers must have done a brilliant
job.

Just a few comments, from a complete outsider to the field.

George Huxtable.

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Paul Hirose said-

>A time of two I have mentioned the B-2 bomber's astro-inertial nav
>system. During my years in the program its accuracy was classified,
>and as far as I know it still is, so I have never given any figures.
>However, a publicly available U.S. military document from the office
>of the Chairman of the Joint Chiefs of Staff, CJCSI 6130.01B, "Master
>Positioning, Navigation, and Timing Plan", has this to say:
>
>[quote]
>
>e. Celestial Navigation. Celestial navigation, as traditionally
>practiced, provides an average error in position of 2 nm. Increased
>flexibility, more accurate calculations, and decreased time to
>solution (fix) can be achieved by performing calculations
>electronically. Accuracies corresponding to 15 to 30 meters on the
>Earth's surface are attained by automated celestial systems, depending
>on the degree of automation. Automated star trackers on spacecraft,
>missile guidance systems, and aircraft provide high-accuracy,
>real-time calibration of position and orientation with respect to the
>absolute inertial reference frame provided by stellar sources.
>Typically a star tracker augments an inertial (or other) guidance
>system. The System to Estimate Latitude and Longitude Astronomically
>(STELLA) is a computer application that automates all of the
>calculations of celestial navigation, including derivation of a fix
>(2D). It is equally useful for determination of the gyro/compass
>error, and supports the necessary planning activities for both
>functions with numeric and graphic displays. STELLA eliminates the
>need for printed tables, log and manual calculations, and can be
>installed on fixed, portable, or lap-top computers for use when
>needed. STELLA has built-in capability for higher accuracy if used in
>conjunction with stabilized or compensated sensors vice hand-held
>instruments.
>
>[Apparently, STELLA is in the DoD section of the USNO's site. Sob.
>Further on in this document:]
>
>(4) Advanced Celestial Navigator
>
>(a) Description. Using fully-developed space-tested astro-trackers,
>the advanced celestial navigator (ACN) will provide day and night
>celestial navigation in partially obscured skies. The highly sensitive
>charged coupled devices, operating in the near infrared, will be able
>to define angles to celestial bodies to within 1 arcsecond, a
>sixty-fold improvement over the current hand-held visual system.
>Celestial fixes to within 30 meters will be common. For aircraft,
>altitudes to 100 feet are realizable.
>
>(b) Mission to be Enhanced Through This Technology. When closely
>coupled to the INS, the ACN will provide stabilization for both
>position and platform alignment. For high-flying aircraft, the ACN/INS
>combination provides a nearly all-weather, unjammable, and precise
>navigation system should GPS be denied.
>
>[end quote]
>
>Here is the complete document (300 k PDF file):
>
>https://www.peterson.af.mil/GPS_Support/documents/masternav_timing_plan.pdf

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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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